Natural thickeners represented by xanthan gum, guar gum, and the like, semi-synthetic thickeners represented by hydroxyethylcellulose, carboxymethylcellulose, and the like, and synthetic thickener represented by carboxyvinyl polymer, polyethylene oxide, and the like have been widely used as hydrophilic thickeners.
Since a carboxyl-group-containing hydrophilic polymer such as a carboxyvinyl polymer exhibits an excellent thickening property even when used at a small amount, the hydrophilic polymer has been very properly used as a thickener, a dispersing agent, or an emulsion stabilizing agent, which is useful in various fields of industry including cosmetics, toiletries, and the like. The carboxyvinyl polymer is a polymer of a water-soluble ethylenically unsaturated monomer containing a carboxyl group and a vinyl group such as an acrylic acid, and may be cross-linked by an oil-soluble crosslinking agent such as pentaerythritol triallyl ether, etc.
In the conventional art, a carboxyl-group-containing hydrophilic polymer used as a hydrophilic thickener has been often prepared by a precipitation polymerization method using an organic solvent. In the precipitation polymerization method, polymer particles, which are insoluble in the organic solvent and precipitate along with the progress of the polymerization, are collected. The collected polymer particles are used as the thickener. Meanwhile, in a method of preparing a water-absorbent resin, the use of reversed-phase suspension polymerization has been studied (Patent Literature 1).